Water heating devices designed to substantially raise the temperature of a body of water, and in some cases generate steam, are known to be prone to deposits of scale forming on the heat source. Commonly affected devices include steam generation devices such as a boiler, as well as conventional water heating devices such as a kettle.
Scale is formed when dissolved solids in the water, such as sulphates or carbonates of calcium and magnesium, are deposited as the water is turned to steam. The layers of scale create an insulating layer around the heat source and reduce the energy efficiency and speed of the water heating process. Furthermore, the insulating layer of scale can cause the heat source to accumulate excess heat so that the temperature of the working components exceeds that required for safe and reliable operation.
Another problem associated with scale is that small fragments of the scale will become detached during steam generation and entrained in the steam flow. In the example of a steam iron, these small fragments of scale are deposited on the garment causing the garment to become dirty.
Scale is typically removed from the heat source by cleaning with a weak acid or by physically scraping off the scale. Both options involve effort and expense and require the steam generation process to be postponed.
Alternatively, it is possible to prevent scale formation by chemically treating water to remove dissolved solids. Ion exchange methods are commonly employed to reduce total dissolved solids, wherein a resin impregnated with sodium ions is arranged to exchange the sodium ions with ions from the dissolved solids in the surrounding water. Disadvantageously, this method requires an additional process and supporting equipment to carry out, which can increase the cost and complexity of steam generation.
Traditional steam generation devices require that the heating element is entirely submerged by the water source such that, in equilibrium of the system, the heating element and scale layer are maintained at a constant temperature. More recent technologies have emerged that generate steam by dripping water onto a heated surface, causing a sudden temperature fluctuation of the plate and scale layer. The temperature fluctuation causes mechanical stress in the scale, which if greater than the scale's tensile strength, causes the scale to break up. The scale is then more easily removed by rinsing or physically scraping the heated surface.
Water dripped onto a heated surface forms a film and migrates across the surface due to the plate surface conditions and the surface tension of the water. This leads to an uneven and unpredictable distribution of water and therefore an uneven and unpredictable distribution of scale. In areas where water pools or gathers, thicker deposits of scale are formed that are harder to break up.
A soleplate for a steam iron is disclosed in U.S. Pat. No. 4,091,551. The soleplate disclosed in the document comprises a plate inclined at an angle to the horizontal having an upper and lower end. The soleplate further comprises a heating element to heat the soleplate, including the plate, and a water inlet arrangement for dispensing water onto the plate. The soleplate is heated to a temperature which evaporates the water.